The present invention relates generally to optical fiber coupler structures, and more particularly to a novel fiber structure and optical coupler configuration for reducing the reflection of optical signals at a fiber end.
The invention applies principally to optical systems wherein a common fiber is used for both transmit and receive functions. The fiber end may interface with a lens, mirror or similar optical element that collects light transmitted from the fiber and directs it as required by the function of the optical system. The element then may also collect received light and concentrate it on the same fiber end for propagation along the fiber to an optical detector to accomplish the receive function.
A significant and persistent problem exists with this type of optical system. As transmitted light arrives at the exit surface of the fiber, a portion (typically .about.4%) of the light is reflected back into the fiber according to Fresnel's laws of reflection and propagates back through the fiber to the optical detector in the same manner as the desired receive signals. This transmitter leakage is often greater in magnitude than the desired receive signals, and may mask the desired signals and/or saturate the optical detector so that no receive signals can be detected. The purpose of this invention is to greatly reduce this undesired fiber exit surface reflected light.
The invention describes an optical fiber structure for reducing undesirable reflections of optical signals at the fiber end. Light rays may propagate along a fiber in either direction in any of three general forms, viz., as axial rays along the optical axis, as paraxial rays essentially parallel to the optical axis, or as skew rays transmitted in helical fashion around the optical axis. Light reflections from an interface at a fiber end may return as interference down the fiber length to a receiver or other optical system to which the fiber is connected. In order to ensure that most fiber end reflected optical signals propagated along the fiber are reflected out of the acceptance cone of the fiber, the fiber end through which the signals are transmitted is ground and polished at preselected angle to prevent paraxial rays from achieving perpendicular or near perpendicular reflection from the fiber end. Attenuation of skew rays is ensured by providing a flattened portion of preselected length and depth at the angled end of the fiber. The fiber may interface at the angled end with an optical coupler of suitable structure for accommodating transmission of an optical signal across the angled fiber end at an angle to the transmission axis of the fiber.
It is therefore a principal object of the invention to provide an improved optical fiber and coupler structure.
It is another object of the invention to provide an optical fiber structure and optical fiber coupler configuration for reducing the reflection of optical signals at the fiber end.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.